Vibrating screens are used, for example, in crushing plants for sorting material into different classes according to the particle size. A screen comprises a screening element, which may be, for example, a sieve, a mesh or a grate, whose openings are passed by pieces of the material to be screened which are smaller than a given fraction size. The screen may comprise several screening elements, which may be placed, for example, on top of each other. Advantageously, all the screening elements of the screen are moved by joint aggregates. E.g. the publication WO 02/00359 describes such a prior art vibrating aggregate being suitable for moving a screening element of a vibrating screen. Typically, the vibrating screen moves along an elliptical path. In solutions of prior art, the elliptical movement and the direction of the vibrating screen are produced by spring suspension and by rotating two or more eccentric masses in different phases.
There is also so called “directional-action mechanical vibrator” that can be used for moving a screening element. In this kind of solution there is two or more moving masses but these are usually used for balancing each other and the vibration is formed mechanically i.e. not by means of the inertia forces caused by movement of the masses. This type of vibrator is described in publication U.S. Pat. No. 4,287,779.
Feeders as well as vibrating screens are used in crushing plants for the purpose of sorting the material, and feeders are also used for feeding the material to be crushed into the crusher. In the feeder, the smaller fraction that should not to be fed into the crusher, is separated out. With respect to its function, the feeder resembles the vibrating screen.
In solutions of prior art wherein the elliptical movement path is generated by means of the inertia forces, two or more drive shafts are needed for rotating eccentrically rotating masses. The shafts must be synchronized by means of a gearing or cogged belts, which makes the structure complex.